Metal sensing through fluorescence quenching: the role of carbon quantum dots as fluorescent probes

Carbon dots (CDs) are a class of carbon nanoparticles which possess interesting properties such as high fluorescence, high solubility and size in the nanometre range. This thesis work focused on the role of CDs as fluorescent probes for harmful metals sensing through the phenomenon of fluorescence quenching, in which the intensity of the fluorescence emission of the CDs is reduced following a bonding with a metal particle. For this application, glucose-based CDs were synthesized through hydrothermal method in an autoclave at 200°C, followed by purification by dialysis with the use of a 5kDa cut-off membrane, lasting 72h. Lastly, the resulting nanoparticles were thermally treated in an oven at 60°C for several days. Characterization was done with several methods, namely FTIR, Raman spectroscopy, AFM analysis, UV-Vis spectroscopy and fluorescence spectroscopy. The first four were used to determine the quality, morphology and chemical identity of the CDs, while the last one was adopted to study the fluorescent behaviour both for a solution containing only the CDs and for a solution in which metal particles were also inserted, and quenching was observed. The first solution was comprised of 0.7mg/mL of CDs in purified water, and fluorescence was collected for excitation wavelengths ranging from 280nm to 340nm, since the absorption profile of the CDs was observed to belong in that range. For what concerns metal containing solutions, two different metals were used to test fluorescent quenching: Fe(III) and Cr(III). Solutions with different concentrations of each metal were prepared and analysed, ranging from 25ppm to 500ppm. The quantity of CDs and water in these solutions was chosen equal to the solution containing only CDs, in order to achieve a clean comparison in between the fluorescence spectra. Fluorescent quenching was analysed at an excitation wavelength of 300nm, since the related fluorescence peak was observed to be the most responsive across the emission spectrum of the CDs. A satisfying and detectable quenched response was registered even down to 25ppm which, by comparison between the fluorescence spectrum of the CDs-only solution and the spectrum of the metal containing ones, demonstrated that carbon dots can be used as fluorescent probes for metal sensing.